Thermoelectric elements using metal-insulator transition material

ABSTRACT

Provided is a thermoelectric device including a first electrode, a substrate electrically connected to the first electrode, a thin film on the substrate, and a second electrode on the thin film. The substrate and the thin film may be configured to exhibit a metallic property at a temperature over a critical temperature, thereby having a thermoelectric power of the device that is higher than that of a semiconductor junction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 to Korean Patent Application Nos. 10-2012-0073434 and10-2013-0003890, filed on Jul. 5, 2012 and Jan. 14, 2013, respectively,in the Korean Intellectual Property Office, the entire contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Example embodiments of the inventive concept relate to a thermoelectricdevice, and in particular, to thermoelectric elements using ametal-insulator-transition metal.

Thermoelectric devices, as one of clean energy production technologies,have been suggested to produce electricity from waste heat.Conventionally, the thermoelectric devices have been realized using asemiconductor pn junction. The thermoelectric device may convert thermalenergy into electric energy using Seeback Effect. Alternatively, thethermoelectric device may increase or decrease a temperature thereofusing Peltier Effect, when an external voltage is applied thereto.Thermoelectric efficiency of the thermoelectric devices may be expressedby ZT=(S2s/k)T, where S is a Seeback coefficient, s is electricconductivity, k is thermal conductivity, and T is a measurementtemperature. For example, ZT coefficients of 1 and 2 may representefficiencies of about 10% and 20%, respectively. It has been known thata super lattice structure of Bi2Te3/Sb2Te3 has ZT of 2.5. Junctions madeof ceramics p-Si and n-Si are being commercialized as thermoelectricelements. In the meantime, when a car is running, its radiator may beheated up to a temperature of 200° C. In addition, there is a largeamount of waste heat of 100° C. near boiler. This means that there is anincreasing demand for thermoelectric device efficiently harvesting wasteheat of about 200° C. or less.

SUMMARY

Example embodiments of the inventive concept provide a thermoelectricdevice using a metal-insulator transition (MIT) material.

Other example embodiments of the inventive concept a thermoelectricdevice capable of realizing efficiency higher than that of semiconductorthermoelectric elements.

According to example embodiments of the inventive concepts, athermoelectric device may include a first electrode, a substrateelectrically connected to the first electrode, a thin film on thesubstrate, and a second electrode on the thin film. The substrate andthe thin film may be configured to exhibit a metallic property at atemperature over a critical temperature, thereby having a thermoelectricpower of the device that is higher than that of a semiconductorjunction. In addition, as the result of the metallic property, adifference in work-function between the substrate and the thin film maybe increased with increasing temperature. The substrate and the thinfilm may be configured to exhibit a metal-insulator transition (MIT)metal property, and have more carriers than a semiconductor and have animproved thermoelectric property.

In example embodiments, the substrate may include silicon. The siliconlayer may be doped with conductive impurities. The conductive impuritiesacceptors, which are one of boron or gallium, or donors, which are oneof the arsenic and the phosphorus. A concentration of the acceptors orthe donors in the silicon layer may be about 10¹⁸ cm⁻³. In exampleembodiments, the thin film may include a metal-insulating transition(MIT) material. The MIT material may include a vanadium oxide of VO₂ orV₂O₃. The vanadium oxide of VO₂ exhibits the metallic property at 67°C., and the vanadium oxide of V₂O₃ exhibits the metallic property at 150K.

In example embodiments, the metallic property may be an electricproperty measured from a thermoelectric current and a thermal voltagebetween the MIT material and the substrate.

In example embodiments, the thin film may include a strongly-correlatedmetal. For example, the strongly-correlated metal may include YBa₂Cu₃O₇or MgB₂.

In example embodiments, the thin film include at least one of a)inorganic semiconductor materials with low concentration of holes andinsulating materials, b) organic semiconductor materials with lowconcentration of holes and insulating materials, c) semiconductormaterials with low concentration of holes, or d) oxide semiconductormaterials with low concentration of holes and insulating materials, andthe inorganic semiconductor materials may include at least one ofoxygen, carbon, III-V compound semiconductors, II-VI compoundsemiconductors, transition metal materials, rare earth materials, orlanthanum-based materials

In example embodiments, the first electrode may be provided on or belowthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will be more clearly understood from the followingbrief description taken in conjunction with the accompanying drawings.The accompanying drawings represent non-limiting, example embodiments asdescribed herein.

FIGS. 1 and 2 are sectional views illustrating a thermoelectric deviceaccording to example embodiments of the inventive concept.

FIGS. 3 and 4 are graphs showing metal-insulator transitioncharacteristics of VO₂ and V₂O₃.

FIGS. 5 and 6 are graphs showing a metallic property of a crystallinesilicon substrate.

FIG. 7 is a graph showing thermoelectric power characteristics orSeebeck coefficient of VO₂ MIT thin film.

FIG. 8 is a graph showing a change in workfunction over temperature.

FIG. 9 is a graph showing power factors of a substrate and thin films.

FIGS. 10 and 11 are graphs showing thermoelectric current and thermalvoltage characteristics of a thermoelectric device with MIT thin film ofVO₂.

FIGS. 12 and 13 are graphs showing thermoelectric current and thermalvoltage characteristics of a thermoelectric device with MIT thin film ofV₂O₃.

FIG. 14 is a graph showing electric power characteristics of theconventional pn junction of crystalline silicon.

It should be noted that these figures are intended to illustrate thegeneral characteristics of methods, structure and/or materials utilizedin certain example embodiments and to supplement the written descriptionprovided below. These drawings are not, however, to scale and may notprecisely reflect the precise structural or performance characteristicsof any given embodiment, and should not be interpreted as defining orlimiting the range of values or properties encompassed by exampleembodiments. For example, the relative thicknesses and positioning ofmolecules, layers, regions and/or structural elements may be reduced orexaggerated for clarity. The use of similar or identical referencenumbers in the various drawings is intended to indicate the presence ofa similar or identical element or feature.

DETAILED DESCRIPTION

Example embodiments of the inventive concepts will now be described morefully with reference to the accompanying drawings, in which exampleembodiments are shown. Example embodiments of the inventive conceptsmay, however, be embodied in many different forms and should not beconstrued as being limited to the embodiments set forth herein; rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the concept of example embodimentsto those of ordinary skill in the art. In the drawings, the thicknessesof layers and regions are exaggerated for clarity. Like referencenumerals in the drawings denote like elements, and thus theirdescription will be omitted.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Like numbers indicate like elementsthroughout. As used herein the term “and/or” includes any and allcombinations of one or more of the associated listed items. Other wordsused to describe the relationship between elements or layers should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” “on” versus “directlyon”).

It will be understood that, although the terms “first”, “second”, etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of example embodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises”, “comprising”, “includes” and/or “including,” if usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Example embodiments of the inventive concepts are described herein withreference to cross-sectional illustrations that are schematicillustrations of idealized embodiments (and intermediate structures) ofexample embodiments. As such, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, example embodiments of theinventive concepts should not be construed as limited to the particularshapes of regions illustrated herein but are to include deviations inshapes that result, for example, from manufacturing. For example, animplanted region illustrated as a rectangle may have rounded or curvedfeatures and/or a gradient of implant concentration at its edges ratherthan a binary change from implanted to non-implanted region. Likewise, aburied region formed by implantation may result in some implantation inthe region between the buried region and the surface through which theimplantation takes place. Thus, the regions illustrated in the figuresare schematic in nature and their shapes are not intended to illustratethe actual shape of a region of a device and are not intended to limitthe scope of example embodiments.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments of theinventive concepts belong. It will be further understood that terms,such as those defined in commonly-used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the relevant art and will not be interpreted in anidealized or overly formal sense unless expressly so defined herein.

FIGS. 1 and 2 are sectional views illustrating a thermoelectric deviceaccording to example embodiments of the inventive concept.

Referring to FIGS. 1 and 2, a thermoelectric device may include asubstrate 10, a first electrode 20, a thin film 30, and a secondelectrode 40.

The first electrode 20 and the second electrode 40 may include a metal,such as gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten(W), or zinc (Zn). The first electrode 20 may be provided on or belowthe substrate 10. The first electrode 20 and the second electrode 40 maybe electrically connected to each other via the substrate 10 and thethin film 30. The first electrode 20 may be spaced apart from the thinfilm 30 and the second electrode 40. The second electrode 40 may beprovided on the thin film 30.

The substrate 10 may include p- or n-type crystalline silicon exhibitinga conductive property. For example, the substrate 10 may be doped withacceptors (e.g., boron or gallium) to have a p-type conductivity. Inother example embodiments, the substrate 10 may be doped with donors(e.g., arsenic or phosphorus) to have an n-type conductivity. Theacceptors or the donors in crystalline silicon may be doped to have alow concentration of about 10¹⁸ EAcm⁻³. This may correspond to acritical value for MIT to occur in a material, and for a Mott material,the critical carrier concentration, n, is (0.25/a_(H))^((−1/3))=10¹⁸cm⁻³, where a_(H) represents a radius of hydrogen atom.

The thin film 30 may include at least one of metal-insulator transition(MIT) materials, such as VO₂ and V₂O₃. For example, VO₂ and V₂O₃ mayhave a metallic property at a temperature higher than criticaltemperatures thereof.

FIGS. 3 and 4 are graphs showing metal-insulator transitioncharacteristics of VO₂ and V₂O₃.

Referring to FIGS. 3 and 4, a resistance of a metal-insulator transitionmaterial may be abruptly decreased at its critical temperature. VO₂ is atypical MIT material exhibiting a large resistance change at a criticaltemperature of about 67° C. or 340K and exhibiting a metallic propertyat a temperature over the critical temperature, as shown in FIG. 3. V₂O₃is a MIT material exhibiting a large resistance change at a criticaltemperature of about 150K and exhibiting a metallic property at atemperature over the critical temperature, as shown in FIG. 4. The thinfilm 30 may have a thickness of about 90 nm or more. The thin film 30and the substrate 10 may constitute a pn junction at a criticaltemperature or more. The junction made of the thin film 30 and thesubstrate 10 may have a work-function difference much higher than thatof a semiconductor-based pn junction. In addition, the substrate 10 andthe thin film 30 may exhibit a metallic property of a thermoelectricpower higher than that of the semiconductor-based pn junction, at acritical temperature or more.

FIGS. 5 and 6 are graphs showing a metallic property of a crystallinesilicon substrate.

Referring to FIGS. 5 and 6, for the substrate 10 of crystalline silicon,an electrical resistance, as shown in FIG. 5, and a thermoelectric poweror a Seebeck coefficient, as shown in FIG. 6, may increase along with atemperature, similar to electrical characteristics of typical metals.This is a completely new feature of the crystalline silicon substrate 10that was so far unknown, in that silicon has been known to have electricresistance decreasing with an increase of temperature. In other words,this MIT metal property may result from the fact that the crystallinesilicon is doped with completely different carriers.

FIG. 7 is a graph showing thermoelectric power characteristics orSeebeck coefficient of VO₂ MIT thin film 30.

Referring to FIG. 7, the thin film 30 on the substrate 10 exhibits highthermoelectric power or high Seebeck coefficient S at a temperature overthe MIT transition temperature. The Seebeck coefficient S is about 20-40for typical metals, but was 150 or more at a temperature over 360K forthis.

FIG. 8 is a graph showing a change in workfunction with temperature.

Referring to FIG. 8, in thermoelectric elements according to exampleembodiments of the inventive concept, a difference in work-functionbetween the substrate 10 and the thin film 30 was increased withincreasing temperature.

FIG. 9 is a graph showing power factors of the substrate 10 and the thinfilm 30.

Referring to FIG. 9, the power factor was much higher than the knownvalue. Power factor may be given by an equation P=S²/r, where S and rare Seebeck coefficient and resistivity, respectively. A magnitude ofthe power factor may be used to evaluate characteristics of thin-filmtype thermoelectric elements. According to example embodiments of theinventive concept, a metallic substrate with p-type carriers and ametallic MIT thin film with n-type carriers may be combined to form a pnjunction thermoelectric element. In certain embodiments, two metalliclayers with carriers of the same type may be combined to form ajunction-type thermoelectric element, in which a difference inworkfunction between two metallic layers increases with increasingtemperature. Such a junction-type thermoelectric element may be providedin a form of, for example, n-n junction a p-p junction.

FIGS. 10 and 11 are graphs showing thermoelectric current and thermalvoltage characteristics of a thermoelectric device with the MIT thinfilm 30 of VO₂.

Referring to FIGS. 10 and 11, thermoelectric currents were significantlyincreased at about 70° C. Thermal voltages were increased for highresistance and decreased for low resistance. Here, the substrate 10 wasdoped with p-type conductive impurities to have a low concentration ofabout 10¹⁸EAcm⁻³. The VO₂ thin film 30 had a thickness of about 100 nmand an area of 5×5 mm². Referring to FIGS. 10 and 11, powers (i.e., I×V)of samples 6 and 7 were 1.12 mW/cm² and 1.00 mW/cm², respectively, at ahigh temperature of 100° C.

Although not shown, at a temperature of 270° C., the thermoelectricpower of the VO₂ MIT device was measured to have about 240 mW/cm², andthis value was much higher that 0.441 mW/cm², known as the highest powerso far, of Bi—Sb—Te thin film. This result shows that MIT thermoelectricelements using MIT metal is much more effective than semiconductorthermoelectric elements.

FIGS. 12 and 13 are graphs showing thermoelectric current and thermalvoltage characteristics of a thermoelectric device with the MIT thinfilm 30 of V₂O₃.

Referring to FIGS. 12 and 13, thermoelectric currents were significantlyincreased at about 85° C. Thermal voltages were increased withincreasing temperature. Similarly, the thin film 30 had an area of 5×5cm². Powers (i.e., I×V) of samples 4 and 5 were 6.93 mW/cm² and 8.68mW/cm², respectively, at a high temperature of 100° C.

FIG. 14 is a graph showing electric power characteristics of theconventional pn junction of crystalline silicon. For a conventionalsilicon pn junction, a thermoelectric power was 0.03 mW/cm² at 535K and0.19 mW/cm² at 596K. An n-type silicon thin film provided on a p-typesilicon substrate was used for the silicon pn junction. The siliconsubstrate and the silicon thin film had an area of 30×20 mm². Asdescribed above, the junction made of silicon and MIT material had athermoelectric power of 1.0 mW/cm² or more.

In this sense, the thermoelectric device according to exampleembodiments of the inventive concept is definitely superior to thesilicon thermoelectric device in terms of thermoelectric powercharacteristics. In example embodiments, to achieve an increased currentand an increased voltage and to harvest waste heat, a plurality ofthermoelectric devices may be connected in parallel or in series to eachother.

Referring back to FIGS. 1 and 2, the thin film 30 may include asuperconductivity material, e.g., YBa₂Cu₃O₇, MgB₂, which is known as astrongly-correlated metal exhibiting a metallic property at a lowtemperature lower than the room temperature.

Alternatively, the thin film 30 may include at least one of a) inorganicsemiconductor materials with low concentration of holes and insulatingmaterials, b) organic semiconductor materials with low concentration ofholes and insulating materials, c) semiconductor materials with lowconcentration of holes, or d) oxide semiconductor materials with lowconcentration of holes and insulating materials. In example embodiments,the inorganic semiconductor materials may include at least one ofoxygen, carbon, III-V compound semiconductors, II-VI compoundsemiconductors, transition metal materials, rare earth materials, orlanthanum-based materials.

According to example embodiments of the inventive concept, a thin filmof MIT material may be provided on a doped silicon substrate. The dopedsilicon substrate and the MIT thin film may exhibit a thermoelectricpower and/or a metallic property that is superior to a semiconductor pnjunction, at a high temperature (e.g., over the room temperature). Forexample, the junction with the silicon substrate and the MIT thin filmmay have a high thermoelectric power of 1.0 mW/cm² or more, while atypical silicon-based pn junction has a low thermoelectric power of 0.1mW/cm² or less.

In this sense, the thermoelectric device according to exampleembodiments of the inventive concept can be operated with higherefficiency than conventional thermoelectric elements using asemiconductor pn junction.

While example embodiments of the inventive concepts have beenparticularly shown and described, it will be understood by one ofordinary skill in the art that variations in form and detail may be madetherein without departing from the spirit and scope of the attachedclaims.

What is claimed is:
 1. A thermoelectric device, comprising: a firstelectrode; a substrate disposed directly on the first electrode; a thinfilm disposed directly on the substrate; and a second electrode disposeddirectly on the thin film, wherein the thin film comprises ametal-insulating transition (MIT) material, and wherein the substrateand the thin film are configured in such a way that majority carriersthereof have types different from each other.
 2. The device of claim 1,wherein the substrate is doped with conductive impurities, and whereinthe conductive impurities include acceptors, which are one of boron orgallium, or donors, which are one of arsenic and phosphorus.
 3. Thedevice of claim 1, wherein the MIT material comprises a vanadium oxideof VO₂ or V₂O₃.
 4. The device of claim 3, wherein a metallic property isan electric property measured from a thermoelectric current and athermal voltage between the thin film and the substrate.
 5. The deviceof claim 3, wherein the vanadium oxide of VO₂ exhibits a metallicproperty at 67° C.
 6. The device of claim 1, wherein the MIT materialincludes a vanadium oxide of VO₂, and wherein the thin film has athickness of about 100 nm.
 7. The device of claim 1, wherein the MITmaterial includes a vanadium oxide of V₂O₃, and wherein the thin filmincludes a top surface having an area of about 25 mm².
 8. The device ofclaim 1, wherein the substrate comprises silicon doped with conductiveimpurities and a concentration of the conductive impurities is about10¹⁸ cm⁻³.
 9. A thermoelectric device comprising: a substrate; a firstelectrode disposed directly on the substrate; a thin film disposeddirectly on the substrate and separated from the first electrode; and asecond electrode disposed directly on the thin film, wherein the thinfilm comprises a metal-insulating transition (MIT) material, and whereinthe substrate and the thin film are configured in such a way thatmajority carriers thereof have types different from each other.
 10. Thedevice of claim 9, wherein a bottom surface of the first electrode iscoplanar with a bottom surface of the thin film.
 11. The device of claim9, wherein the substrate comprises silicon that is doped with impuritiesand a concentration of the impurities is about 10¹⁸ cm⁻³.
 12. The deviceof claim 11, wherein the MIT material includes a vanadium oxide of VO₂,and wherein the thin film has a thickness of about 100 nm.
 13. Thedevice of claim 9, wherein the MIT material includes a vanadium oxide ofV₂O₃, and wherein the thin film includes a top surface having an area ofabout 25 mm².